This invention relates to the cogeneration of electricity and carbon dioxide (CO.sub.2) with low NO.sub.x combustion of vaporizable liquid fuels.
Cogeneration systems have received considerable attention in recent years. Many processes have been proposed for the cogeneration of electricity and saleable refrigeration or heating steam. A few processes have been suggested for the cogeneration of electricity and CO.sub.2. By and large, cogeneration has been based on fuel combustion associated with a gas turbine. However, the operation of gas turbines has two disadvantages: combustion is carried out with a large amount of excess air which reduces thermal efficiency, and the products of combustion or flue gas has a high content of nitrogen oxides (NO.sub.x) and other atmospheric pollutants.
For example, U.S. Pat. No. 4,528,811 to Stahl shows several cogeneration systems involving gas turbines. Stahl points out that gas turbines are normally operated with about 300% excess air over the stoichiometric requirement; he proposes to curtail the use of excess air by recycling the combustion products to his two-portion combustor. However, such recycling is at the expense of compressing the expanded combustion products back to the pressure in the combustor. Moreover, such recyling does not curtail the formtion of NO.sub.x and other pollutants. To the contrary, Stahl's divided combustor which admittedly generates high temperatures may well increase the formation of NO.sub.x, carbon monoxide (CO) and unburned hydrocarbons, particularly inasmuch as Stahl uses only 10% excess air in the combustion zone of his combustor.
U.S. Pat. No. 4,797,141 to Mercader et al describes a process for recovering heat contained in the combustion gases from an engine or turbine that drives an electric generator as well as heat given off by the engine or turbine. The recovered heat is used in the absorption separation of CO.sub.2 from the combustion gases. However, the combustion gases from internal combustion engines are no less polluted than those from gas turbines.
In short, there is an urgent need to overcome the handicaps of cogeneration systems involving fuel combustion and gas turbines, namely, the use of a large amount of excess air and the emission of high levels of atmospheric pollutants.
Accordingly, a principal object of this invention is to avoid the use of gas turbines in cogeneration systems by substituting steam turbines therefor.
A related principal object is to generate steam for the steam turbines by burning a liquid fuel in a manner that minimizes the emission of NO.sub.x and other gaseous pollutants.
A further object is to limit the amount of excess air used in the combustion to yield a flue gas of increased CO.sub.2 content.
Another important object is to recover CO.sub.2 from the flue gas by absorption and desorption effected with some of the generated steam.
Still another important object is to liquefy the recovered CO.sub.2 with refrigeration produced with energy derived from the generated steam.
These and other features and advantages of the invention will be evident from the description that follows.